FlyWithLua can be downloaded here, https://forums.x-plane.org/index.php?/files/file/38445-flywithlua-ng-next-generation-edition-for-x-plane-11-win-lin-mac/ .
You place the script in the FlyWithLua plugin's Scripts folder. If you load straight into the Saab you will then find a FlyWithLua command with the description "Toggles the LES Saab 340A parking brake on and off." which you can bind.
Don't worry, I'm sure LES will address this and the missing GS indication soon.